Optimize tombstone creation for ExpiringColumns

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Description

The goal of this ticket is to reduce the amount of tombstones created from ExpiringColumns.

Currently tombstones will always stay a full gc_grace time, which is not neccessary for ExpiringColumns. We only need to ensure that ExpiringColumn and tombstone together live as long as gc_grace. If the ExpiringColumn's TTL>=gc_grace then we can create an already gcable tombstone and drop that instantly.

My initial proposal was to use the ExpiringColumns creation-timestamp as deletiontime for the tombstone, but Sylvain pointed out that we should not mix local and client timestamps. So I changed it to this:

The goal of this ticket is to reduce the amount of tombstones created from ExpiringColumns.

Currently tombstones will always stay a full gc_grace time, which is not neccessary for ExpiringColumns. We only need to ensure that ExpiringColumn and tombstone together live as long as gc_grace. If the ExpiringColumn's TTL>=gc_grace then we can create an already gcable tombstone and drop that instantly.

My initial proposal was to use the ExpiringColumns creation-timestamp as deletiontime for the tombstone, but Sylvain pointed out that we should not mix local and client timestamps. So I changed it to this:
{code}
public static Column create(ByteBuffer name, ByteBuffer value, long timestamp, int timeToLive, int localExpirationTime, int expireBefore, IColumnSerializer.Flag flag)
{
if (localExpirationTime >= expireBefore || flag == IColumnSerializer.Flag.PRESERVE_SIZE)
return new ExpiringColumn(name, value, timestamp, timeToLive, localExpirationTime);
// the column is now expired, we can safely return a simple tombstone
return new DeletedColumn(name, *localExpirationTime-timeToLive*, timestamp);
// return new DeletedColumn(name, localExpirationTime, timestamp); // old code
}
{code}

The goal of this ticket is to reduce the amount of tombstones created from ExpiringColumns.

Currently tombstones will always stay a full gc_grace time, which is not neccessary for ExpiringColumns. We only need to ensure that ExpiringColumn and tombstone together live as long as gc_grace. If the ExpiringColumn's TTL>=gc_grace then we can create an already gcable tombstone and drop that instantly.

My initial proposal was to use the ExpiringColumns creation-timestamp as deletiontime for the tombstone, but Sylvain pointed out that we should not mix local and client timestamps. So I changed it to this:
{code}
public static Column create(ByteBuffer name, ByteBuffer value, long timestamp, int timeToLive, int localExpirationTime, int expireBefore, IColumnSerializer.Flag flag)
{
if (localExpirationTime >= expireBefore || flag == IColumnSerializer.Flag.PRESERVE_SIZE)
return new ExpiringColumn(name, value, timestamp, timeToLive, localExpirationTime);
// the column is now expired, we can safely return a simple tombstone
return new DeletedColumn(name, localExpirationTime-timeToLive, timestamp);
// return new DeletedColumn(name, localExpirationTime, timestamp); // old code
}
{code}